1. Field of the Invention
The present invention relates to a ball screw apparatus and an electric power steering apparatus.
2. Description of the Related Art
It is well known that there is a rack assisted electric power steering apparatus (EPS) having a hollow cylindrical shaft penetrated by a rack shaft and rotatably driven by a motor to transform rotational movement of the hollow cylindrical shaft to axial movement of the rack shaft by a ball screw apparatus, therefore assisting rotation of a steering wheel.
The conventional ball screw apparatus of the EPS includes a plurality of balls rotating in a spiral rotating way formed by facing a nut side screw groove formed on an inner peripheral surface the ball screw nut to a shaft side screw groove formed on an outer peripheral surface of the rack shaft. Each ball is put between the shaft side screw groove and the nut side screw groove in the rotating way to be rotated in the rotating way by load due to a relative rotation of the ball screw nut relatively to the rack shaft. The ball screw apparatus includes a returning way connecting one end and the other end of the rotating way. Each ball is circulated through the returning way from one of end to the other end of the rotating way after it has been rotated in the rotating way. The ball in the returning way does not receive any load on the contrary of receiving load in the rotating way. The ball dipped up to the returning way from the rotating way is moved to a ball circulating direction in the returning way by being pushed by next adjacent ball.
The prior ball screw apparatus constructed above transforms the rotation of the ball screw nut to the axial movement of the rack shaft by endlessly circulating each ball rotating in the rotating way through the returning way. The rack assisted EPS transmits the assisting force to the steering system by rotatably driving the ball screw apparatus with using the motor and transmitting the rotational torque to the axial pushing force to rack shaft.
The prior ball screw apparatus includes a circulating member as a deflector having function to dip up the ball from the rotating way in relative to a mounted hole penetrated the ball screw nut in radius direction, and function of re-discharge of the ball to the rotating way as disclosed in Tokkai 2010-71411. In the deflector type ball screw apparatus, however, it appears some steps on a jointing point between the rotating way and the returning way on a basis of figure accuracy of circulating member and assembling accuracy to assemble to the ball screw nut 13 etc. It generates any vibration or noise by clogging the ball circulated in the rotating way and the returning way due to the steps. It is therefore disclosed a prior art that the steps are machined to be ground to minimum at the jointing point between the rotating way and the returning way as disclosed in Tokkai-Hei 11-270648.
It is disclosed in another prior ball screw apparatus of Japanese Patent 3381735 that a ball nut (20) is extended in axial direction when load acting on the ball nut (20) is increased to the axial direction thereby to contact the ball (30) with a ball screw groove (24) located to be apart far from a fixed flange (25). By this construction the ball screw apparatus in the Japanese Patent 3381735 has the axial load evenly over full stroke of the screw groove because all of the ball contact with any portion of the screw groove even predetermined high load acts on the ball nut (20). Numerals in ( )indicate that of members in the Japanese Patent 3381735.
However, it should be happened in the ball screw apparatus that moving direction of the ball is rapidly changed near the jointing point between the rotating way and the returning way when the ball is dipped up from the rotating way to be entered into the returning way. In adding to this point, it should be also happened that each ball is rotated with the load in the rotating way and the load acted on the ball does not act in the returning way. Therefore, when the ball is discharged into the rotating way from the returning way, two points should be simultaneously happened that the moving direction is suddenly changed and the load acted on the ball is increased rapidly so that clogging the ball is easily happened near the jointing point between the rotating way and the returning way.
The ball screw apparatus in the Japanese Patent 3381735 has the phenomenon that every ball contacts with every point of the screw groove by the axial load having over the predetermined amount to the ball nut. It tends for the ball screw apparatus in the Japanese Patent 3381735 that the ball is easily clogged by rapidly increased load when the ball is discharged from the returning way. It tends also for the ball screw apparatus in the Japanese Patent 3381735 that a number of the ball contributing to transmit torque is changeable in dependent on small or large amount of the axial load acting on the ball nut. Especially when the axial load acting on the ball nut is small, it tends to be happened that efficiency of transmitting the torque is worse because of small numbers of the ball contributing to transmit the torque.